Electrochemical fuel cells convert fuel such as H2 or methanol, and oxidant such as air or O2, to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) in which an electrolyte in the form of an ion-exchange membrane is disposed between an anode layer and a cathode layer. These electrode layers are made from porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support to the membrane which typically thin and flexible.
The MEA contains a catalyst, typically comprising finely comminuted platinum particles disposed in a layer at each membrane/electrode layer interface, to catalyze the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
It is generally believed that the large-scale practical application of fuel cells will be difficult to achieve if the expensive platinum-based cathode catalyst, which carries out the oxygen reduction reaction (ORR), cannot be realized either with significantly reduced catalyst loading or with another efficient, low-cost, and durable catalyst. As a result, the development of non-precious metal catalysts (NPMCs) with high oxygen reduction reaction activity and improved durability has become a major focus area of polymer electrolyte fuel cell (PEFC) research as a way of reducing the PEFC cost.
NPMCs have been developed over the years. The synthesis typically includes a heat-treatment step wherein precursors of nitrogen, carbon, and transition-metals are combined at an elevated temperature to produce active sites for oxygen reduction [1, 2].
Factors affecting catalyst performance are believed to include the types of nitrogen precursors and the transition metal(s) used. Some NPMCs for oxygen reduction were derived from heteroatomic polymers such as polyaniline (PANI) and polypyrrole (PPy) [3,4], and were synthesized by heat-treating a hybrid precursor material containing PANI or PPy, polymerized in situ onto conventional carbon black and nanotubes in the presence of iron and cobalt species. The Fe and Co species of these catalysts are believed to contribute differently to the active sites for oxygen reduction. The Co species appear to promote nitrogen doping into graphitized carbon by forming abundant pyridinic structures, which are presumed active ORR sites. With an onset oxygen reduction potential of approximately 0.8 V, this species has electrochemical properties similar to that exhibited by metal-free N—C catalysts. By contrast, Fe species are presumed to participate at an active site by coordinating the pyridinic and pyrrolic nitrogen atoms, similar to Fe—N4[5]. The latter site is more active and gives rise to an onset potential higher than 0.9 V that is less overpotential for ORR [3]. The Fe—Co hybrid catalyst exhibits oxygen reduction reaction activity, and also long-term performance durability that is believed to be due to a stabilizing role of Co.
A need remains for better non-precious metal catalysts that approach, meet, or exceed the performance and durability precious metal catalysts such as platinum.
An object of this invention is an active and durable oxygen reduction reaction catalyst.
Another object of this invention is a method for preparing an active and durable oxygen reduction reaction catalyst.